Field of the Invention
The present invention relates to record medium and apparatus useful in high density storage of information by optical writing and/or reading, and more particularly to improved configurations for optical disc assemblies and cooperating optical disc write/read apparatus.
Description of the Prior Art
The currently preferred optical disc technology employs disc elements with spiral or concentric tracks of minute (e.g., on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning tracks on the disc with an equivalently small beam of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with the same size or only slightly larger, but still very tightly focused, radiation (e.g. light) spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out reflected from the recorded disc.
In order to write and read information in the form of such minute markings, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relation between the writing or reading optical system, and the optical disc record surface must be stringently maintained both smooth and flat.
One approach to achieve requisite smoothness and flatness has been to form the disc substrate of glass with a ground and polished surface. This requires a time consuming and costly fabrication procedure. Another approach is to mold a plastic disc substrate with a highly finished surface and apply a surface smoothing sub-layer. However, it is extremely difficult to mold such plastic discs having adequate surface characteristics with a good yield; and this support fabrication method is also relatively costly.
Another approach for meeting smoothness, flatness and protective requirements is disclosed in commonly-assigned, copending U.S. patent application Ser. No. 264,313 filed May 18, 1981 in the names of F. F. Geyer and E. M. Leonard. In that approach an improved optical disc assembly adapted for high density storage of information comprises (i) a flexible, disc-shaped support carrying a record layer; (ii) a transparent disc cover sheet opposing the record layer, and (iii) an annular retaining ring for holding the support and cover sheet in circumferentially-symmetric tension. The support material may be held in a relatively low tension by the retaining ring and tensioned to an operating tension when drawn over a rotatable annular locating surface on the disc assembly receiving structure of the optical disc write/read apparatus. Thus the disc assembly can normally be kept with the support material in low "storage" tension, and used with the support material in higher "operating" tension to enhance flatness.
The disc-shaped support must be accurately centered on the locating surface so that the information tracks are closely concentric with the axis of rotation of the locating surface. The greater the degree of eccentricity of the mounted disc assembly, the more difficult it is for the write/read optics to follow the information trails.